FAILED: patch "[PATCH] mm/huge_memory: fix NULL pointer deference when splitting" failed to apply to 6.12-stable tree

The patch below does not apply to the 6.12-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.12.y
git checkout FETCH_HEAD
git cherry-pick -x cff47b9e39a6abf03dde5f4f156f841b0c54bba0
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2025120102-geranium-elevator-ca86@gregkh' --subject-prefix 'PATCH 6.12.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From cff47b9e39a6abf03dde5f4f156f841b0c54bba0 Mon Sep 17 00:00:00 2001
From: Wei Yang <[email protected]>
Date: Wed, 19 Nov 2025 23:53:02 +0000
Subject: [PATCH] mm/huge_memory: fix NULL pointer deference when splitting
folio

Commit c010d47f107f ("mm: thp: split huge page to any lower order pages") introduced an early check on the folio's order via mapping->flags before proceeding with the split work.

This check introduced a bug: for shmem folios in the swap cache and truncated folios, the mapping pointer can be NULL. Accessing mapping->flags in this state leads directly to a NULL pointer dereference.

This commit fixes the issue by moving the check for mapping != NULL before any attempt to access mapping->flags.

index 2f2a521e5d68..6cba1cb14b23 100644
@@ -3619,6 +3619,16 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 	if (folio != page_folio(split_at) || folio != page_folio(lock_at))
 		return -EINVAL;
 
+	/*
+	 * Folios that just got truncated cannot get split. Signal to the
+	 * caller that there was a race.
+	 *
+	 * TODO: this will also currently refuse shmem folios that are in the
+	 * swapcache.
+	 */
+	if (!is_anon && !folio->mapping)
+		return -EBUSY;
+
 	if (new_order >= folio_order(folio))
 		return -EINVAL;
 

@@ -3659,18 +3669,6 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 		gfp_t gfp;
 
 		mapping = folio->mapping;
-
-		/* Truncated ? */
-		/*
-		 * TODO: add support for large shmem folio in swap cache.
-		 * When shmem is in swap cache, mapping is NULL and
-		 * folio_test_swapcache() is true.
-		 */
-		if (!mapping) {
-			ret = -EBUSY;
-			goto out;
-		}
-
 		min_order = mapping_min_folio_order(folio->mapping);
 		if (new_order < min_order) {
 			ret = -EINVAL;

From: Wei Yang <[email protected]>

[ Upstream commit cff47b9e39a6abf03dde5f4f156f841b0c54bba0 ]

Commit c010d47f107f ("mm: thp: split huge page to any lower order pages") introduced an early check on the folio's order via mapping->flags before proceeding with the split work.

This check introduced a bug: for shmem folios in the swap cache and truncated folios, the mapping pointer can be NULL. Accessing mapping->flags in this state leads directly to a NULL pointer dereference.

This commit fixes the issue by moving the check for mapping != NULL before any attempt to access mapping->flags.

---
 mm/huge_memory.c | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

index d68a22c729fb3..2065374c7e9e6 100644
@@ -3404,6 +3404,16 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
 	if (new_order >= folio_order(folio))
 		return -EINVAL;
 
+	/*
+	 * Folios that just got truncated cannot get split. Signal to the
+	 * caller that there was a race.
+	 *
+	 * TODO: this will also currently refuse shmem folios that are in the
+	 * swapcache.
+	 */
+	if (!is_anon && !folio->mapping)
+		return -EBUSY;
+
 	if (is_anon) {
 		/* order-1 is not supported for anonymous THP. */
 		if (new_order == 1) {

@@ -3466,13 +3476,6 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
 		gfp_t gfp;
 
 		mapping = folio->mapping;
-
-		/* Truncated ? */
-		if (!mapping) {
-			ret = -EBUSY;
-			goto out;
-		}
-
 		min_order = mapping_min_folio_order(folio->mapping);
 		if (new_order < min_order) {
 			VM_WARN_ONCE(1, "Cannot split mapped folio below min-order: %u",

2.51.0